System and method for acquisition of signals

ABSTRACT

A system for processing a received signal having at least one code applied thereto, said received signal having a frequency, said system comprising: first correlator circuitry arranged to correlate the received signal with a first code to provide an output; second correlator circuitry arranged to correlate the received signal with a second code to provide an output, wherein said first code and said second code are different; and processing means for processing together the outputs of said first and second correlator circuitry to cancel said frequency.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a system and method for the acquisitionof signals and, in particular, but not exclusively to the acquisition ortracking of spread spectrum navigation signals such as used in globalnavigation satellite systems GNSS. The present invention also relates tothe acquisition or tracking of broadcast pseudo random codes, inparticular codes transmitted as part of a satellite navigation signalsuch as a GPS or GNSS signal.

BACKGROUND OF THE INVENTION

One example of a global navigation satellite system is the GlobalPositioning System GPS. The global position system (GPS) uses satellitesorbiting the earth in known orbit paths with accurately known positions.These satellites transmit signals which can be received by a receiver onearth. Using signals received from four or more satellites, the receiveris able to determine its position using trigonometry. The signalstransmitted by the satellite comprise pseudo-random codes. The accuracyof the determination of position is dependent on factors such as therepetition rate of the code, the components of the receiver andatmospheric factors.

The accuracy of measurement can be improved if the signals from morethan four satellites are taken into account. GPS receivers can receivesignals from up to twelve satellites at a time.

GALILEO is a European initiative for a global navigation satellitesystem which provides a global positioning service. It has been proposedthat GALILEO be interoperable with GPS and GLONASS, the two other globalsatellite navigation systems. It should be appreciated that the termGNSS is used in this document to refer to any of these globalpositioning systems.

GALILEO has a system of thirty satellites, twenty-seven operationalsatellites with three operational in-orbit spares. The proposedfrequency spectrum for GALILEO has two L-bands. The lower L-band,referred to as E5a and E5b, operate in the region of 1164 MHz to 1214MHz. There is also an upper L-band operating from 1559 MHz to 1591 MHz.

In GPS, the frequency signals themselves are called the carriers. Sincethe carriers are pure sinusoids, they have two binary codes modulatedonto them. The two binary codes are the C/A (coarse acquisition) codesand the P (precise) code.

Information on the coordinates of the satellite is also sent within themessages broadcast by the satellite and is modulated onto the carriers.

The coarse/acquisition (C/A code) was originally used as a coarseposition measurement signal or as an acquisition code in order to assistlocking onto the phase of the precise code. However, the C/A code is nowused for acquisition and for position tracking. The C/A code is apseudo-random (PN) binary code which consists of 1023 elements or chipsthat repeat every millisecond.

The C/A code is a Gold code. The cross correlation property of a Goldcode is such that the correlation function between two differentsequences is low. In other words, when a received signal is correlatedwith the correct C/A code the desired signal can be easily distinguishedfrom other signals which appear as background noise.

In GPS and GALILEO, signals are broadcast from satellites which includethese pseudo random codes which are processed at a receiver to determineposition data. The processing involves first determining the relativeoffset of the received codes with locally generated versions of thecodes (acquisition) and then determining the position once the relativeoffset is determined (tracking). Both acquisition and tracking involvecorrelating received signals with a locally generated version of thepseudo random codes over an integration period.

In spread spectrum systems, acquisition is difficult because it is twodimensional (frequency and time). A further difficulty is that becausethe signals are much weaker inside as compared to outside, it is muchmore difficult to acquire signals indoors. In particular, the indooroperation of GNSS requires the reception of signals attenuated by atleast 20 dB from the outdoor equivalents.

The number of cells in the time domain is for example 2046. The numberof cells in the frequency domain is 20 for outdoors at 1 KHz bandwidthor 2000 for indoors at 10 Hz bandwidth. This latter number of cells canbe reduced to 20 with a temperature controlled crystal oscillator TCXO.This means that the total number of cells to be searched is 2046 times20 i.e. around 40,000. For outdoors, each cell takes one millisecond andfor indoors, each cell would take 100 milliseconds because of the weakersignal strength. This results in a search time of 40 seconds foroutdoors or 4000 seconds for indoors, on a single correlator.

This problem traditionally is addressed by using parallelism in thefrequency domain, for example sixteen fast Fourier transform channels orby parallelism in the time domain, using parallel correlators. Toachieve parallelism requires faster clocks or more hardware which isdisadvantageous. Additionally, more hardware or faster clocks requireincreased power.

In any event, the fundamental limit is the stability of the referenceclock which prevents bandwidth reduction to the degree required forindoor sensitivity.

As already mentioned the indoor signals can be attenuated by at least 20dB from their outdoor equivalents. To increase the sensitivity by twentydB for the indoor signals means integrating for a hundred times longer.However, this is difficult to achieve because as the coherentintegration period is extended, the bandwidth of the channel isnarrowed. This in turn requires many more searches to be carried out andeventually the stability of the reference oscillator becomes a limitingfactor as a signal appears to wander from one frequency to another, evenbefore acquisition is completed. This results in a spreading of theenergy, preventing further gain.

In addition, the modulation method used may provide a limit on theintegration time.

Thus there are problems in performing integration with such signals. Theintegration time is limited by the accuracy of a local clock and thefrequency shifts caused by relative motion of the satellite andreceiver.

It is an aim of embodiments of the invention to address one or more ofthese problems.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, an aspectof the present invention is to provide a system for processing areceived signal having at least two channels, each of said channelshaving the same carrier frequency and the same data carried thereby,said first and second channels having respective first and second codesapplied thereto, said first and second codes being different, saidsystem comprising correlation circuitry arranged to correlate thereceived signal with a third code to provide an output, said third codebeing derived from a function of said first and second codes; andprocessing means for providing an output for at least one of: trackingof said signal; acquisition of said signal; and extracting informationfrom said signal.

According to a second aspect of the present invention there is provideda receiver for receiving from a source a signal having at least twochannels, each of said channels having the same carrier frequency andthe same data carried thereby, said first and second channels havingdifferent spreading codes applied thereto.

According to a third aspect of the present invention there is provided atransmitter for transmitting a signal having at least two channels, eachof said channels having the same carrier frequency and the same datacarried thereby, said first and second channels having differentspreading codes applied thereto.

According to a fourth aspect of the present invention there is provideda system for processing a received signal having at least a first codeand a second code applied thereto, said received signal having afrequency, said system comprising first correlator circuitry arranged tocorrelate the received signal with a third code derived from said firstand second code to provide an output; second correlator circuitryarranged to correlate the received signal with a fourth code to providean output, said fourth code being derived from said first and secondcode, wherein said third code and said fourth codes are different; andprocessing means for processing the outputs of said first and secondcorrelator circuitry together.

According to a fifth aspect of the present invention there is provided asystem for processing a received signal having at least one code appliedthereto, said received signal having a frequency, said system comprisingfirst correlator circuitry arranged to correlate the received signalwith a first code to provide an output; second correlator circuitryarranged to correlate the received signal with a second code to providean output, wherein said first code and said second code are different;and processing means for processing the outputs of said first and secondcorrelator circuitry together to cancel said frequency.

According to a sixth aspect of the present invention there is provided amethod for processing a received signal having at least one code appliedthereto, said received signal having a frequency, said method comprisingthe steps of correlating the received signal with a first code;correlating the received signal with a second code, wherein said firstcode and said second code are different; and processing together theresults of the first and second correlating steps to cancel saidfrequency.

According to a seventh aspect of the present invention method forprocessing a received signal having at least two channels, each of saidchannels having the same carrier frequency and the same data carriedthereby, said first and second channels having respective first andsecond codes applied thereto, said first and second codes beingdifferent, said method comprising correlating the received signal with athird code to provide an output, said third code being derived from afunction of said first and second codes; and processing said output toprovide an output for at least one of: tracking of said signal;acquisition of said signal; and extracting information from said signal

According to an eighth method for processing a received signal having atleast a first code and a second code applied thereto, said receivedsignal having a frequency, said method comprising correlating thereceived signal with a third code derived from said first and secondcode to provide an output; correlating the received signal with a fourthcode to provide an output, said fourth code being derived from saidfirst and second code, wherein said third code and said fourth codes aredifferent; and processing the outputs of said first and secondcorrelating steps together.

In one receiver embodying the invention, two or more signals derivedfrom a transmitter having coherent carrier signals are multiplied by twoor more local codes to produce signals of coherent carrier butindependent noise. These signals are multiplied together to produce forexample an output signal comprising sum and difference signals and thenintegrated over an integration period.

By multiplying signals together that are derived from the transmitterand which are coherent, the integration period is no longer limited byfrequency drift either due to local clock variation or due to Dopplershift between the transmitter (satellite) and receiver. This is becausethe carrier signals are coherent. In a sense, one of the signals derivedfrom the transmitter is used as a “local” oscillator signal for theother.

In one embodiment, the two or more local codes comprise first and seconddifferent codes corresponding to different codes within the signalsderived from the transmitter. In the GALILEO example, these codes aretwo different codes relating to the LlB and LlC signals. These aretransmitted on the same carrier, are multiplied in the receiverseparately by the respective local b and c codes, and then these signalsmultiplied together.

In this embodiment, the received signal may comprise two different codesbroadcast at the same carrier frequency.

In another embodiment, the two or more codes are derived from one code.Preferably there are two codes that are derived from one codecorresponding to a code in the signal broadcast from one satellite. Thetwo derived codes are orthogonal. This is achieved by taking a codesequence and sampling, for example, taking even samples and odd samplesto generate two respective orthogonal codes. Such codes have zero crosscorrelation when synchronised.

The incoming received signal from one satellite comprising a mixture ofsignal and noise is multiplied by these two orthogonal codes derivedfrom a local version of the code in the satellite signal. The resultingsignals when multiplied together have zero frequency offset as thesignals are derived from a single satellite signal. In this embodiment,the signal comprises a single code broadcast at a carrier frequency.

The techniques may be used in acquisition or tracking of signals. Thesecond embodiment in which the local code is halved into odd and evensamples is particularly useful for code acquisition.

The first embodiment is particularly beneficial for the GALILEO systemin which two separate codes are broadcast on the same carrier. Thesecond embodiment is particularly beneficial for the GPS system in whicha single code is broadcast on a carrier.

The embodiments of the invention may be embodied in a method ofacquiring or tracking signals, in a semiconductor integrated circuit forprocessing, acquiring or tracking received broadcast signals or in areceiver for such.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; and the term“circuitry” may mean any device, system or part thereof that controls atleast one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcircuitry may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and as to how thesame may be carried into effect, reference will now be made by way ofexample only to the accompanying figures, in which like referencenumerals represent like parts, and in which:

FIG. 1 shows the despreading codes used in embodiments of the invention;

FIG. 2 shows schematically an arrangement embodying the presentinvention;

FIG. 3 shows a third arrangement embodying the present invention;

FIG. 4 shows Tricode Hexaphase signalling as used in the prior art;

FIG. 5 shows Tricode Hexaphase signalling as used in embodiments of theinvention;

FIG. 6 shows a timing diagram of the signalling shown in FIG. 5;

FIG. 7 shows an arrangement incorporating the first embodiment of theinvention;

FIG. 8 shows a modification to the arrangement of FIG. 7 where there isonly a primary code on the L1C channel;

FIG. 9 shows an embodiment of the present invention where the tworeceived signals have the same data thereon; and

FIG. 10 shows a further embodiment for processing two signals having thesame data thereon.

DETAILED DESCRIPTION

FIGS. 2 through 10, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented in any suitably arranged system operable to acquire or trackbroadcast pseudo random codes, in particular codes transmitted as partof a satellite navigation signal such as a GPS or GNSS signal.

The embodiments described are in relation to a GNSS receiver for GNSSsignal acquisition and tracking. Embodiments of the present inventionare particularly applicable to the GALILEO system and other satellitenavigation systems such as the GPS system operated by the US governmentor any other global navigation satellite system.

It should be appreciated that whilst embodiments of the presentinvention have been used particularly in the context of acquisition ofsignals for global navigation satellite systems, embodiments of thepresent invention can be used for the acquisition of any other signals.

Embodiments of the present invention are particularly applicable to theacquisition of spread spectrum signals.

It should be appreciated that embodiments may be implemented to providea software equivalent to the circuitry shown in the embodimentsdescribed hereinafter. Embodiments may be implemented in hardware only.Particularly preferred embodiments of the invention are implemented inhardware and software.

The acquisition circuitry can be incorporated in any suitable devicewhich is to provide a positioning functionality. The device can be aportable device or part of a larger device. For example embodiments ofthe present invention can be incorporated in satellite navigationdevices, communication devices such as mobile communication devices forexample mobile phones or any device requiring position information. Thesatellite navigation devices can be stand alone devices or devicesincorporated in various different forms of transport such as cars,trains, aeroplanes, balloons, ships, boats, trucks, helicopters or anyother form of transport.

Preferred embodiments of the present invention, which will now bedescribed, are incorporated in an integrated circuit. However, it shouldbe appreciated that alternative embodiments of the present invention maybe at least partially implemented in discrete circuitry.

The GALILEO L1 BOC (Binary offset carrier) (1, 1) signals offer thepotential for effective indoor operation as needed for mass-marketreceivers. However achieving that economically, especially acquisitionat low signal strengths, remains a challenge. Architectures alreadyproposed for GPS come into their own with the vastly improvedcross-correlation performance of the GALILEO signal.

The BOC features of the signal are generally aids to accuracy, notacquisition, which they make significantly more difficult. However thereare also attributes of the signals that make acquisition significantlyless difficult. The L1C channel has a carrier, a primary spreading codec and a secondary code. Thus there is no data on the Llc channel. Thisremoves the limit of coherent integration presented by the unknown databits. If time can be adequately known, then the secondary codes presentno problem to the acquisition engine, while greatly improving thecross-correlation performance by increasing the effective code-lengthfrom 4 to 100 milliseconds. The secondary codes are may be knownrelatively short codes.

If there is no access to time, then the secondary codes allow a tieredacquisition, saving processing MIPs (million instructions per seconds)over a single layer code of that length. This may require a significantamount of memory for intermediate results, giving great advantage toreceivers with a software element, which can use memory flexibly.

However a major benefit in the signal format is the presence of twosignals at the same frequency, synchronized perfectly at the carrierlevel. Additionally for acquisition purposes they are also synchronizedat symbol level. The Carrier synchronization is guaranteed at thesatellite by the phase angle definitions for the combination of the L1a/b/c signals, so that the phase angle specifications ensure absolutefrequency lock.

Given two carriers at precisely the same frequency, even though thatfrequency is relatively unknown due to satellite Doppler, user Dopplerand reference oscillator error, advantage of this can be taken in theacquisition phase. This can be achieved by combining the signals in thereceiver, thus effectively eliminating the unknown frequency, andseparating the time domain acquisition from the frequency domainacquisition. In outdoor GPS, there may be a 20 KHz frequency search in 1KHz bins and 2046 time domain bins making 40,000 searches to perform.Using these same parameters when the frequency and time searches can beseparated means 2046 plus 20 searches. About 95% saving in search timeand MIPS can be achieved.

The GALILEO code length is not yet published, since the change from BOC(2,2) to BOC(1,1). However the same percentage saving is expected, butmay be even more significant because the baseline processing load isabout 8 times higher, due to the BOC (×2) and the expected code length(×4).

The above methodology gives a huge advantage, but is only viable when acoherent integration period of 4 milliseconds (one symbol) isappropriate, possibly extended by subsequent non-coherent integration.This is because it requires both L1b and L1c signals, and the L1b signalhas data symbols on it. For deeply attenuated indoor signals, advantageof the data-free L1c signal to extend the coherent integration periodcan be taken. Thus the frequency search is required, so optimization ofthe time domain search is even more important.

If there is access to a time reference at the 4-millisecond level, it ispossible to predict the secondary code and thus eliminate it. This meansthat only 8192 time searches are required. For a totally unaidedacquisition the secondary-code cannot be predicted, meaning that the L1csignal would appear to be no better than the data-modulated L1b signal.The data modulation is unknown but the secondary code, while unknownbetween 25 possible phases, does only have 25 possibilities, no matterif the desired integration period is 100 ms or 1 second. Thus it ispossible to perform integrations over single 4 millisecond epochs, andaccumulate them into 25 independent sets of results. These 25 sets ofresults are then post-processed against the 25 possible phases of thesecondary code. One of the results yields a true coherent integration ofthe chosen length.

There are of course complexities. The coherent integration cannot be solong that it is incompatible with the width of the frequency bin, whichitself must be wide enough to be compatible with the stability of thereference oscillator over the desired period. A set is a fullcorrelation i.e. 8192 results. Even if carefully scaled to stay in asingle byte, this is about 8 kbytes per set, 200 kbytes total. Thisamount of memory for intermediate results might not be viable on ahardware machine, where the memory could not be reused. However on amixed hardware/DSP (digital signal processor)/CPU (computer processingunit) type implementation, it may be available for a short time beforebeing reallocated either to tracking channels or to the CPU forapplication use.

A more efficient implementation may limit the number of intermediateresults stored. However this may mean that an identity is stored witheach result, extending its size. For example storing 100 results only,at 3 bytes each, would be 300 bytes per set, about 8 Kbytes total.Optimal performance would come from a triangular matrix, where say 8000results are stored from the first epoch, then 4000+4000, then2000+2000+2000+2000 etc, discarding earlier possibilities based on atemporary incoherent combination, or possibly running the secondarycodes possibilities in real time. However this may involve excessivecomplexity, certainly for a full hardware implementation, making thehardware-DSP-CPU architecture preferable again.

Embodiments of the present invention greatly reduce the parallelismrequired for a given sensitivity, saving silicon and power, and alsovastly speeding up acquisition by separating the time domain acquisitionfrom the frequency domain, meaning n+m cells to search, not n.x.m, a 95%saving in the above examples.

In addition, it is resilient to unstable reference clocks, allowing thenoise bandwidth to be reduced to extreme levels without missing thesignal.

On a two-signal system, such as GALILEO LlB/C, the signal can beacquired without precise knowledge of frequency by combining theinformation from each signal.

They have identical Doppler shift from satellite movement, and identicalDoppler shift from user motion, so after they have been separatelyde-spread and integrated to the level of bandwidth required, the twocomplex samples from each signal can be multiplied together andintegrated again. The carrier element of each signal will have preciselythe same frequency, therefore the product will be at zero Hz, and thuscan be synchronously integrated for 4 milliseconds (the period of thedata symbol). Thus the LlB (data) signal can be demodulated with noprecise knowledge of the frequency.

This results in a (data×secondary code) stream, which can be multipliedby the known secondary code to recover the data, initially synchronisingby looking for (preamble×secondary code). This is as described above.

FIG. 7 shows circuitry for implementing the above described embodiment.

A signal is input to a first mixer 500. The first mixer 500 receives anintermediate frequency signal at a frequency of 4 fo. fo is the nominalfrequency i.e. the basic chipping rate which in the GALILEO system is1.023 MHz. The input frequency is mixed with a frequency of 4 fo from alocal oscillator. The output of the first mixer will however still havea small frequency component due to the Doppler shift effect and othereffects as mentioned previously. This is input to a second mixer 502.Estimation of the Doppler shift is used to generate an output from anumerically controlled oscillator which is input to the second mixer tofurther remove the frequency component. There may still however, be asmall frequency component remaining.

The output of the second mixer 502 is input to a third mixer 504 and afourth mixer 506. The third mixer 504 receives an input of the LlBspreading code i.e. b. The output of the third mixer is input to a firstcorrelator 508. The signal input to the fourth mixer 506 is mixed withthe LlC primary spreading code i.e. c. It is not mixed with thesecondary code. The output of the fourth mixer is input to a secondcorrelator 510. The correlators are in the form of synchronous,integrate and dump low pass filters. Thus, the output of the firstcorrelator is the received signal correlated with the spreading code ofthe L1B channel whilst the output of the second correlator 510 is thereceived signal correlated with the L1C primary spreading code. Each ofthe correlators provides an I signal and a Q signal output. The Isignals output by the first and second correlators 508 and 510 are inputto a fifth mixer 512 whilst the Q outputs of the first and secondcorrelators 508 and 510 are input to a sixth mixer 514. The output ofthe fifth and sixth mixers 512 and 514 are input to an adder 516.

The output of the adder 516 is input to a detection block 518, theoutput of which is the data of the L1B channel and the secondary code ofthe L1C channel. This output can be multiplied by the known secondarycode to recover the data.

The output of the adder 516 is also input to a seventh mixer 520 whichalso receives the output of the detector 518. The mixed output providedby the seventh mixer 520 is input to a low pass filter 522 whichprovides an amplitude output for code tracking control. The data is thusremoved from the signal which is at zero Hertz. This means that theresulting dc signal can be integrated for a longer period than the datasymbol period.

FIG. 8 shows a modification to the arrangement of FIG. 7. In thismodified embodiment, the signal on the L1B channel comprises thecarrier, the spreading code b and data. This is as described in theembodiment of FIG. 7. However, the L1C channel comprises the carrier andthe spreading code c. In other words, there is no secondary spreadingcode as described in the relation to the embodiment of FIG. 7. It shouldbe appreciated that those elements which operate in the same or similarway to that shown in FIG. 7 are referenced by the same referencenumerals. However, since the L1C channel only has the primary spreadingcode, the output of the second correlator 510 can be put into IQ mixcircuitry 524 which multiplies the current IQ pair by the complexconjugate of the previous IQ pair and carries out an accumulateoperation. The output of the IQ mix circuitry 524 is input to a low passfilter 522 which provides an accumulate function. The magnitude of theresult is the signal power. The phase of the result is frequency. Thus,the output is noise free and the frequency output is used for trackingcontrol.

It should be appreciated that the output of the detector 518 is simplythe data.

In a second embodiment, the same task can be carried out on a singlesignal system such as GPS. To do this the GPS CA-code for a givensatellite is split alternately into two components.

Reference is made to FIG. 1 which shows how the GNSS C/A code for agiven satellite is used in embodiments of the invention. The first line10 is the pseudo-random number PRN code. This code consists of a seriesof 1s and 0s. The second line 20 shows the meaning of the PRN code.Thus, every “1” of the PRN code means “1” and every “0” means “−1”.

In embodiments of the present invention the PRN code is splitalternatively into two components 22 and 24. The sum of the twocomponents gives the original code at any point. At any point onecomponent is the same as the original C/A code and the other componentis 0. The PRN code is effectively divided up into an even code 22 andodd code 22. The code of the PRN code is counted from zero so the firstelement of the PRN code is “even” code. In other words the odd and evencodes are interleaved to provide the original code.

In alternative embodiments of the invention, the code can be split intotwo codes in any appropriate way. The code can be split for example sothat the first m bits are in one code and the remaining n bits are inthe other code. Alternatively, instead of alternative single bits,alternative k bits can be used, where k is an integer. For example if kis 2, then the first 2 bits are in one code and the third and fourthbits are in the other code and so on. The pattern for dividing the bitsmay be regular or may be pseudorandom. In some embodiments of theinvention, the single code can be split into three or more codes.

It should be appreciated that there may be a faster sampling rate thanone per code chip. However, all of the samples of a particular chip areactive in the same code, and zero in the other code.

The even and odd codes are used to despread the GNSS signals. Thisyields two quasi-independent outputs after integration. Each is 3 dBdown in signal strength from the conventional signal. A complex multiplyis then performed between the two resulting streams and the integrationis carried out again. This integration can be carried out without limitbecause in each stream there are identical carriers and identical BPSK(Binary phase shift keying) data. After multiplication and integration,the carrier and data are eliminated. The noise is thus reduced as thenoise band width is now minimal (that is the reciprocal of theintegration period).

This yields only a DC value proportional to the power of the signalwhich can be used in a traditional early/late tracker. Because of this,the integration is self synchronous; no knowledge of data edges isrequired. Thus, integration for 1 second to a 1 Hz bandwidth would yieldextreme sensitivity, unachievable in a system with conventional clocksthat randomly wander by more than that in the search time involved(2046) searches.

Reference is now made to FIG. 2 which shows circuitry for implementingan embodiment of the present invention. It should be appreciated that inpractice, implementation of the embodiment of the present invention maybe achieved by a computer program, i.e. by a software implementation.Embodiments of the invention can be implemented in a mixture of hardwareand software. For example the circuitry up to the correlators may be inhardware whilst the circuitry downstream of the correlators may beimplemented by software.

This same circuitry may be used to implement the first embodiment wheretwo different codes are transmitted. The two different transmitted codeswould be used instead of the odd and even codes of this embodimentillustrated in FIG. 2.

The arrangement shown in FIG. 2 shows the real part of the signal (the Ipart). It should be appreciated that components shown are also arrangedto handle the imaginary part of the signal, i.e. the Q part. Thecomponents are thus complex components. The information carried on the Iand Q channels may be the same or different.

A first mixer 50 is provided. The first mixer receives an intermediatefrequency signal at a frequency of 4 fo. fo is 1.023 MHz i.e. thenominal frequency. This signal contains the information carried on theL1c channel. The L1c channel is in the L1 frequency range of 1563MHz-1587 MHz. The primary code length may be is 8184 bits whilst thesecondary code may be of the order of 25 bits. It should be appreciatedthat embodiments of the invention can be carried out on any othersuitable signal.

This signal is mixed with a pure frequency signal of 4 fo from a localoscillator. This produces a difference signal i.e. the carrier componenthas been removed from the L1C signal. This difference signal is outputto a second mixer 52. The output of the first mixer will generally stillhave a small frequency component due to the effect of Doppler shift. Theapproximation of the Doppler shift is provided by a numericallycontrolled oscillator NCO. Thus, any remaining frequency component ofthe signal due to Doppler shift has been removed by the second mixer 52.The output of the second mixer is input to a third mixer 54 as well asto a fourth mixer 56. There will be generally a small frequency element,generally at a still lower frequency due to error in the NCO signal.

The third mixer 54 also receives the even code whilst the fourth mixerreceives the odd code. The output of the third mixer 54 is input to afirst correlator 58 while the output of the fourth mixer 56 is output toa second correlator 60. The first correlator 58 correlates the receivedsignal with the even code whilst the second correlator correlates thereceived signal with the odd code. The first and second correlators arelow pass filters.

The outputs of the first and second correlators are input to a fifthmixer 62. The fifth mixer 62 provides three outputs. The correspondingoutputs of the correlators processing the Q signal are input to thismixer 52. The first output is IoddIeven+QoddQeven which input to anintegrator 64. This provides long term integration, which is coherentintegration. The output is a noise free amplitude output for codetracking control. The remaining small frequency component is cancelledby this operation.

The fifth mixer 62 also provides the output IoddIeven−QoddQeven andIoddQeven±QoddIeven. Iodd and Qodd represent the output of the firstcorrelator path whilst Ieven and Qeven represent the output of thesecond correlator path.

The IoddIeven−QoddQeven signal is for the 2 f Iout signals whilst theIoddQeven±QoddIeven is for the 2 f Qout signal, where f represents thesmall remaining frequency component. These two signals are input to asecond integrator 66 which provides a 2 f frequency tracked outputsignal. This is used to control the NCO so that the f component in thesignal is compensated for and effectively removed.

Reference is now made to FIG. 3 which shows a third embodiment of theinvention. As with the second embodiment, the elements for processingthe real part I of the signal only are shown. In practice, elements forprocessing the imaginary part Q of the signal would also be provided.

The received signal having an approximate frequency of 4 fo IF is inputto a first mixer 150 where the received signal is mixed with a frequencyof 4 fo from a local oscillator. This is similar to the first mixer ofthe second embodiment.

As with the second embodiment, the output of the first mixer 150 isinput to a second mixer 152 where the down converted signal is mixedwith a frequency signal from a NCO to provide an output which has beengenerally compensated to remove the effects of Doppler shift.

The output of the second mixer 152 is input both to a third mixer 154and a fourth mixer 156. The output of the second mixer 152 is mixed withthe L1(b−c) prn (pseudo random number code) by the third mixer 154. Theoutput of the third mixer 154 is input to a first correlator 158. Theoutput of the first correlator is input to first and seconddiscriminators 180 and 182.

The output of the second mixer is mixed with the L1(b+c) prn code by thefourth mixer 156. The output of the fourth mixer 160 is input to thesecond correlator 160. The output of the second correlator is input tothe first and second discriminator 180 and 182.

The first discriminator 180 is able to detect the inputs +2, 0 and −2.If b can have the values +1 or −1 and c can have the values +1 or −1,this means that b+c and b−c will have one of the following results: +2,0 or −2.

Consider the following example: b − c b + c b = data +2 0 +1 0 +2 +1 0−2 −1 −2 0 −1

As can be seen depending on the results of the inputs, it can bedetermined what the b data is. This is done by merging the digitstreams. If either of the b−c and b+c is +2 it is determined that the bdata is +1. Likewise if either of b−c and b+c is −1, it is determinedthat the b data is −1.

The second discriminator 182 detects the inputs 2, −2 as being activeand 0,0 as being inactive. The second discriminator 182 is arranged toinvert one stream (the b+c stream in the example although it could bethe b−c stream in alternative embodiments) and merge active analoguevalues. The results are analogue values as a result of noise.

Consider the following example C = amplitude, b − c b + c secondary code+2.5 0.5 +2.5 .6 +2.7 −2.7 .7 −2.6 +2.6 −2.8 0.2 −2.8

In line 1, b−c is detected to have the value of +2.5 and b+c is detectedto have the value 0.5. It is then determined that the b−c result isactive and the b+c result is inactive and the result is then taken to be2.5. In line 2, b−c is detected to have the value of 0.6 while b+c isdetected to have the value of +2.7. b−c is determined to be inactive.b+c is inverted and the result is taken to be =2.7. This has theadvantage that the noise from the inactive stream is excluded. This cangive a 3 dB SNR benefit. Additionally the combination of full satelliteenergy in the active stream gives a 3-6 dB gain.

Reference is made to FIG. 4 which shows an example of Tricode Hexaphaseaccording to the prior art. The first line shows an example of a b codeof −1 −1 +1 +2 −1 +1 +1. The example c code is shown on line 2 and is −1+1 +1 +1 +1 −1 +1. b−c is therefore 0 −2 0 0 −2 +2 0 as shown on line 3.This is scaled to 0 −1 0 0 −1 +1 0 as shown in line 4. Line 5 shows b−cagain.

The final line shows the results of the inverted correlation 0 +2 0 0 +2+2 0.

The full power is recovered where the inverse of data is equal to thesecondary code otherwise the result is zero. The three states are +, −and disable.

Reference is made to FIG. 5 which shows an example of tricode Hexaphaseused in the third embodiment of the invention. Lines 1 to 3 are as shownin FIG. 4. Lines 5 and 6 are as shown in FIG. 5.

Line 4 show the B+C code and line 7 the b+c signal which is −2 0 +2 +2 00 +2. The inverse correlation signal is shown in line 8 and is +2 0 +2+2 0 0 +2. In other words if b−c is 0, the correlation result is 0 andif the b−c is +or −2 the result is +2.

As can be seen full power is recovered both when data=secondary code andwhen the inverse data=secondary code using the two different paths. Atwo state code only is used. The two states are +2 and 0. In other wordsone of the b+c correlation and the b−c correlation is active when theother is inactive.

Reference is made to FIG. 6 which shows a timing diagram. Signal 1 isthe data and can have states +or −1. This corresponds to B. Signal 2 isthe secondary code and can have the state +or −1. Line 3 shows the B+Ccorrelation which has three states −2, 0, +2. Line 4 shows the B−Ccorrelation and can have the value 2, 0, −2.

It is possible to operate from the b−c correlation alone. The secondarycode symbols can be synchronised for this data. If the signal is +2, thedata is +1 and secondary code is +1. If the signal is −2, then the datais −1, and secondary code is −1. If the signal is 0 then thedata=−secondary code. This last determination can be made aftersynchronisation.

Thus, referring back to FIG. 3, the output of the second discriminatoris thus secondary code symbols and is provided to a secondary codedetector 190 which can be used for unaided outdoor acquisition. Theoutput of the second discriminator is also input to a fifth mixer 184which receives an input 186 of locally generated or stored secondarycodes.

The output of the fifth mixer is input to a correlator 188, again in theform of a low pass filter. This filter is programmable so that theintegration period can be varied. As more information becomes available,the period of the integration can be increased. The output of thecorrelator provides a low noise amplitude and frequency output forfrequency control.

In a further embodiment of the present invention, it is proposed thatthe L1B and L1C channels or signals include identical data. Accordingly,the LlB channel or signal comprises the carrier component, the bspreading code and data. The L1C signal or channel comprises thecarrier, the c spreading code and the same data. The L1C and L1Bchannels are on the same frequencies. Thus, this contrasts with some ofthe previously described embodiments, in that the L1C signal would notinclude a secondary code.

This results in three advantages. Firstly, correlation with b−ccomposite code would always be successful and would use all thesatellite energy in a single channel for maximum sensitivity. The IQmix/dot product algorithm could be used between the independent L1B andL1C correlated channels with simultaneous samples, resulting in anintegration uncorrupted by data transitions or residual rotation foracquisition purposes. The correlation with the b−c composite code couldbe used for frequency tracking and data download.

The third advantage is that the cross-correlation response of the systemwould be greatly improved. The result of the IQ mix algorithm is theproduct of b and c. An interfering signal must cross-correlate with bothof them to provide a significant problem. If the signal had an averageCDMA isolation of 36 dB from each, worst case of 26 dB, there might be52 dB isolation. However, since the codes are synchronous, only one canbe at its worst so it would be more reasonable to say 36+26=62 dB. Theseare the theoretical values. In practice, the improvement may not bequite as large but nevertheless some significant improvement could beexpected.

The first advantage in sensitivity i.e. correlation with the b−ccomposite code is an important advantage. The output of the correlatoris an amplitude not a power. If the signals can be combined coherently,a doubling is thus 6 dB while combining later in the system as in otherproposals is only 3 dB. Additionally, the noise amplitude is half as(b−c) is zero 50% of the code chips with all the energy transmitted onthe L1A channel instead and with b−c=0 in a correlated reference code,half that noise can be rejected.

Benefits achieved by some embodiments of the present invention may beachieved immediately in the 4 millisecond period. The ability toeliminate residual rotation and data bit edges gives the flexibility ofintegration period that adds even more sensitivity when the integrationperiod is extended to longer periods for indoor use, making the systemmuch more sensitive or eliminating problems of crystal/TCXO stability.

It should be appreciated that it is believed that the above describedembodiment provides a better advantage than using a single code. Userscan use b or c or a combination depending on the complexity affordableor necessary in the application. Additionally, the fact that the b−csignal is 50% full power, 50% zero power means that correlating with b−cor b+c provides better signal to noise ratio for the same transmittedaverage power. Using identical data symbols on the L1B and the L1Cchannels gives a further improvement in the signal to noise ratio.

Reference is now made to FIG. 9 which shows circuitry for implementingthis embodiment of the present invention.

In the arrangement of FIG. 9, only the I channel is shown forsimplicity.

The input to the first mixer 540 is 4 foIF. The output of the firstmixer 540 is input to a second mixer 542. The first and second mixersprovide similar functions to for example mixers 500 and 502 of FIGS. 7and 8. The output of the second mixer is input to a third mixer 546. Thesignal which is more or less at base band frequency is mixed with thecode b−c. The output of the third mixer 546 is input to a correlator548. The functions provided by the third mixer 546 and correlator 548are similar to those provided by for example mixer 504 and correlator508 of FIGS. 7 and 8. However, the spreading code used is different.

The output of the correlator 548 is input to IQ mix circuitry 550 whichis similar to that described in relation to FIG. 8. The output of the IQmix circuitry 550 provides a first output which is the data. The secondoutput of the IQ mix circuitry 550 is input to a fourth mixer 552 whichalso receives the first output of the IQ mix circuitry 550. This willremove the data. The output of the fourth mixer 552 is input to a lowpass filter 554 which provides similar function to the low pass filter522 of FIGS. 7 and 8.

In practice, the detect function illustrated in FIG. 8 may be providedby the IQ mix circuitry. The IQ mix circuitry will remove the frequencycomponent. This is because this embodiment is unlike other embodimentswhich have two correlation results to mix together to cancel thefrequency component. The IQ mix circuitry effectively converts thefrequency into phase angle which will be constant for a constantfrequency. Thus the output of the circuitry of FIG. 9 and more preciselythe low pass filter 554 will provide a phase rate output which is usedfor carrier tracking control. This output can be used to control the NCOused to control mixer 542. The low pass filter will also provide anamplitude output as described previously which can be used for codetracking control. There is no data on this output.

As can be seen, the circuitry of FIG. 9 is very much simplified in thata single path is required as the received signal which includes the L1Bchannel and the L1C channel needs to be correlated with the single code,b−c. Thus, this reduces the circuitry required. Additionally, unlike thearrangement where correlation is carried out with b+c as well as b−c, nodecision needs to be made as to which of the two results of thecorrelation need to be used.

The arrangement shown in FIG. 9 is particularly useful for tracking.Depending on the quality of the signals, the circuitry can potentiallybe used for acquisition.

Reference is now made to FIG. 10 which shows circuitry which isparticularly applicable for acquisition where the same data is providedon the LlB and LlC channels.

The arrangement comprises first and second mixers 560 and 562 whichprovide similar functions to for example the first and second mixers ofFIG. 7. Third and fourth mixers 564 and 566 are arranged to receive theoutput of the second mixer 562. The third mixer 564 mixes the signalsfrom the second mixer 562 with the b code.

The fourth mixer 566 mixes the output of the second mixer 562 with the ccode. The output of the third and fourth mixers 564 and 566 are outputrespectively to first and second correlators 568 and 570. Thesecorrelators provide similar functions to those described in relation tofor example FIG. 7.

The outputs of the two correlators are input to a mixer 572 whichprovides a simplified IQ mix function. This is able to cancel thefrequency and the data. The function is simplified as compared to the IQmix circuitry of for example FIG. 8 in that the data has been cancelled.The IQ values from the b channel are multiplied by the IQ values fromthe c channel in the arrangement of FIG. 10. In contrast in thearrangement of FIG. 8, the IQ values are the current pair multiplied bythe previous pair. The IQ mix of FIG. 10 only needs to perform thefunctions IcIb±QbQc. The IQ circuitry does not need to perform thefunctions IcQb±IbQc as the result of these function will be zero.

There is a residual rotation, that is a low or residual frequencycomponent. By using the residual rotation, the NCO can be controlled tocompensate for this frequency component.

The output of the fifth mixer 572 is input to a correlator 574 whichprovides a similar function as, for example, correlator 552 of FIG. 7.This is able to provide long term integration i.e. integrate over arelatively long period.

It should be appreciated that once again all elements exist in I and Q.For simplicity, the real components only are shown for simplicity.

Thus, this embodiment allows a simple removal of data and residualrotation for acquisition purposes. The results achieved can beequivalent to but in certain circumstances better than having asecondary code on one channel and data on the other. There is more poweravailable, there is less noise and there are good cross-correlationcharacteristics.

Furthermore, embodiments of the present invention which use the samedata on both the channels result in low cost simpler hardware.

It should be appreciated that whilst specific embodiments of the presentinvention have been described in the context of the GALILEO L1B and L1Csignals, embodiments of the present invention can be used with any othersuitable signals.

In general, embodiments of the present invention are particularlyapplicable where the two signals in question have the same frequency andare from the same source. Thus, in embodiments of the present invention,the techniques shown allow the unknown i.e. the frequency effects to becancelled out because each of the two signals will have the same valuefor the frequency.

In those embodiments which use b−c, it is possible to use alternativessuch as c−b. It may be possible to use other more complex functions of band c in some embodiments.

It should be appreciated that embodiments of the present invention alsorelate to the provision of a transmitter, for example in a satellitewhich is able to transmit two signals having the same carrier frequency,the same data but different spreading codes. Embodiments of the presentinvention also relate to receivers for receiving signals from asatellite.

The two channels are transmitted at the same time in preferredembodiments of the invention so that it can be assumed that thefrequency, made up of the transmission frequency and the Doppler shift,of the two channels are the same.

Furthermore, embodiments of the present invention have been describedprimarily in the context of obtaining data from satellite navigationsignals. However, it should be appreciated that embodiments of thepresent invention can be used for processing any two or more signalstransmitted from a common source on the same carrier frequency but withdifferent spreading codes.

Embodiments of the invention have been in the context of the acquisitionand tracking of a signal. Particular advantages may be achieved in thecontext of acquisition. It should be appreciated that embodiments of theinvention may be applied to any other suitable signal. It is intendedthat the present invention encompass such changes and modifications asfall within the scope of the appended claims.

1. A system for processing a received signal having at least twochannels, each of said channels having the same carrier frequency andthe same data carried thereby, said first and second channels havingrespective first and second codes applied thereto, said first and secondcodes being different, said system comprising: correlation circuitryarranged to correlate the received signal with a third code to providean output, said third code being derived from a function of said firstand second codes; and processing means for providing an output for atleast one of: tracking of said signal; acquisition of said signal; andextracting information from said signal.
 2. A receiver as claimed inclaim 1, wherein said third code is derived from at least one of the sumand the difference between said first and second codes.
 3. A receiverfor receiving from a source a signal having at least two channels, eachof said channels having the same carrier frequency and the same datacarried thereby, said first and second channels having differentspreading codes applied thereto.
 4. A transmitter for transmitting asignal having at least two channels, each of said channels having thesame carrier frequency and the same data carried thereby, said first andsecond channels having different spreading codes applied thereto.
 5. Asystem for processing a received signal having at least a first code anda second code applied thereto, said received signal having a frequency,said system comprising: first correlator circuitry arranged to correlatethe received signal with a third code derived from said first and secondcode to provide an output; second correlator circuitry arranged tocorrelate the received signal with a fourth code to provide an output,said fourth code being derived from said first and second code, whereinsaid third code and said fourth codes are different; and processingmeans for processing the outputs of said first and second correlatorcircuitry together.
 6. A system as claimed in claim 5, wherein saidthird code comprises a difference between said first and second codes.7. A system as claimed in claim 5, wherein said fourth code comprises asum of said first and second codes.
 8. A system as claimed in claim 5,wherein said processing means is arranged to merge to the outputs ofsaid first and second correlator circuitry to obtain symbols carried onone of first and second channels of said signal.
 9. A system as claimedin any of claims 5, wherein said processing means is arranged to invertthe output of one of said first and second correlator circuitry and tomerge the inverted output of said one correlator circuitry with theoutput of the other of said correlator circuitry to obtain symbolscarried on one of first and second channels of said signal.
 10. A systemas claimed in claim 8, wherein said processing means is arranged todetermine if an output from said first and second correlator circuitryis inactive and if so to exclude said output from said merging.
 11. Asystem as claimed in claim 8, wherein the output of said processingmeans is input to third correlation circuitry for correlating receivedsymbols with locally generated symbols.
 12. A system for processing areceived signal having at least one code applied thereto, said receivedsignal having a frequency, said system comprising: first correlatorcircuitry arranged to correlate the received signal with a first code toprovide an output; second correlator circuitry arranged to correlate thereceived signal with a second code to provide an output, wherein saidfirst code and said second code are different; and processing means forprocessing the outputs of said first and second correlator circuitrytogether to cancel said frequency.
 13. A system as claimed in claim 12,wherein said processing means is arranged to mix the outputs of saidfirst and second correlator circuitry together to cancel said frequency.14. A system as claimed in claim 12, wherein said received signalcomprises at least two channels having said frequency.
 15. A system asclaimed in claim 14, wherein one of said channels has a first known codeapplied thereto and a second of said channels has a second known codeapplied thereto.
 16. A system as claimed in claim 15, wherein said firstcode comprises said first known code and said second code comprises saidsecond known code.
 17. A system as claimed in claim 15, wherein saidfirst code comprises a first function of said first and second knowncodes and said second code comprises a second different function of thefirst and second known codes.
 18. A system as claimed in claim 17,wherein said first function comprises a sum of said first and secondknown codes and said second function comprises a difference of saidfirst and second known codes.
 19. A system as claimed in claim 12wherein at least one of said first and second codes are derived from aknown code applied to said received signal.
 20. A system as claimed inclaim 19, wherein said first code and second code are such that ifcombined the known code is produced.
 21. A system as claimed in claimedin claim 19, wherein said first code comprises even components of saidknown code and said second code comprises odd components of said knowncode.
 22. A system as claimed in any of claims 12, wherein saidprocessing means is arranged to receive an output from said firstcorrelator circuitry and an output from said second correlator circuitryand to perform a multiply operation.
 23. A system as claimed in claim22, wherein processing means is arranged to provide an in-phase andquadrature sum of IaIb±QaQb, where Ia and Qa are provided by said firstcorrelator circuitry and Qb and Ib are provided by said secondcorrelator circuitry.
 24. A system as claimed in claim 23, wherein saidprocessing means comprises an integrator arranged to integrate saidin-phase and quadrature sum to provide an output.
 25. A system asclaimed in claim 24, wherein the output of the integrator comprises a dcvalue proportional to the power of the received signal.
 26. A system asclaimed in any of claims 23, wherein said processing means is arrangedto provide an in-phase and quadrature difference of IaIb−QaQb.
 27. Asystem as claimed in any of claims 22, wherein said processing means isarranged to provide an output of IaQb+QaIb where I is an in-phasecomponent and Q is a quadrature component, Ia and Ib being provided bysaid first and second correlator circuitry respectively and Qa and Qbbeing provided by said first and second correlator circuitryrespectively.
 28. A system as claimed in 26, wherein said processingmeans comprises an integrator, said integrator being arranged to receiveand integrate at least one of the in-phase and quadrature difference andIaQb±QaIb.
 29. A method for processing a received signal having at leasttwo channels, each of said channels having the same carrier frequencyand the same data carried thereby, said first and second channels havingrespective first and second codes applied thereto, said first and secondcodes being different, said method comprising: correlating the receivedsignal with a third code to provide an output, said third code beingderived from a function of said first and second codes; and processingsaid output to provide an output for at least one of: tracking of saidsignal; acquisition of said signal; and extracting information from saidsignal
 30. A method for processing a received signal having at least afirst code and a second code applied thereto, said received signalhaving a frequency, said method comprising: correlating the receivedsignal with a third code derived from said first and second code toprovide an output; correlating the received signal with a fourth code toprovide an output, said fourth code being derived from said first andsecond code, wherein said third code and said fourth codes aredifferent; and processing the outputs of said first and secondcorrelating steps together.